Control apparatus



March 2, 1943. so r 2,312,711

CONTROL APPARATUS Filed March 24, 1939 5 Sheets-Sheet l -FIG.I. 4|

INVENTOR THOMAS R. HARRISON I BYL 1 ATTOR N EY March 2 1943. 1 'r. R. HARRISON 2,312,711

CONTROL APPARATUS Filed March 24, 1939 5 Sheets-Sheet 2 FIG3.

INVENTOR THOMAS R. HARRISON ATTORN EY March 1943. T. R. HARRISON 2,312,711

CONTROL APPARATUS Filed March 24, 1939 5 Sheets-$heet 3 FIGSV 7 I59 I23 158 \NVENTOR I56 THOMAS R.HARRISON 2 511666555865; 55 f gMW I ATTORNEY March 2, 1943. T. R. HARRISON CONTROL APPARATUS 5 Sheets-Sheet 4 Filed March 24, 1939 JLZ FlG.8.

INVENTOR THOMAS R. HARRISON BY MNW ATTORNEY T. R. HARRISON CONTROL APPARATUS March 2, 1943.

Filed March 24, 1939 5 Sheets-Sheet 5 RATE OF RESET RESET ARRESTOR F- m F M M M Q u .m n N m m MA c wH m MM m mm mm m H FIG.I2.

INVENTOR THOMAS R. HARRISON M ATTORNEY Patented Mar. 2, 1943 UNITED STATES PATENT OFFICE CONTROL APPARATUS Thomas R. Harrison, Wyncote, Pa.., assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Application March 24, 1939, Serial No. 263,938

40 Claims.

The present invention relates to electrical control apparatus and more particularly to electrical control apparatus adapted to effect suitable corrective variations in the value of a quantity being controlled, on a departure of the latter from a predetermined normal value, without creating an objectional tendency to unstable control, or hunting.

A general object of the invention is to provide electrical control apparatus adapted on a change in the value of a variable quantity under control to produce a corrective change in the controlling medium to restore the quantity to the desired value in the shortest time possible while avoiding the tendency to hunting.

A specific object of the invention is to provide electrical control apparatus of the character above mentioned having simple and effective means for adjusting it for desirable operation under different conditions of use.

Another specific object of the invention is to provide electrical control apparatus embodying physically" stationary means for compensating for variations in the characteristics of the quantity being controlled.

Another specific object of the invention is to provide electrical control apparatus which is adapted, on a change in the value of a variable quantity being controlled, to produce a relanvely large initial corrective kick or control effect in the controlling medium,

while avoiding the tendency to hunting which would ordinarily result from such a large initial corrective adjustment.

A further and .more specific object of the invention is to provide electrical control apparatus embodying physically stationary means for producing a disproportionately strong corrective effeet in the supply of the controlling medium during the initial stages of a departure of the quantity being controlled from a desired value while avoiding the tendency to hunting which would ordinarily result from such strong initial corrective effect.

Electrically operated controllers embodying the features of the present invention may take widely different forms and are adapted for use for many different purposes. In general they may be used whenever it is desirable to produce a control effect in response to a change in a control condition or quantity such, for example, as temperature, pressure, flow. liquid level, etc., which tends to vary as a result of the control effect produced.

In accordance with the present invention suitable provisions are made to prevent variations in the quantity being controlled, which variations may be due to changes in the effect of the controlling medium, to changes in the characteristics or amount of the quantity being controlled, or to any other variable conditions. On a change in an operating condition, such, for example, as a change in the B. t. u. content of the fuel supplied to a furnace, or to a change in the furnace load, the furnace temperature will tend to vary, but due to the heat inertia thereof, the change in the operating condition will have been existent for some time before it results in a temperature change that is detected by the apparatus em ployed to maintain the furnace temperature at a desired value. When thereafter a correction in the amount of fuel supplied the furnace is made by the apparatus referred to in order to restore the furnace temperatue to the desired value, such correction will not be eifective to immediately restore the desired condition. This lag is also due to the heat inertia of the furnace. Furthermore, if a sufficiently large correction has been made to restore the furnace temperature to the desired value within a reasonably short time and is maintained until that value is reached, the furnace temperature will tend to overshoot that value and subsequent corrective adjustments will result in hunting or oscillation of the furnace temperature about the desired value.

Accordingly, a primary object of the present invention is to provide electrical control apparatus which is adapted to prevent such hunting or oscillation and effect control or regulation at an even given value.

In a preferred form, the present invention includes provisions for effecting a relatively large initial corrective adjustment in the amount of the controlling medium supplied to maintain the desired condition and thereafter, before the condition has returned to that desired value, removing the initially large corrective adjustment whereby the tendency to hunting, which would ordinarily result from such large initial corrective adjustment, is avoided.

The preferred form of the invention also includes provisions for effecting a so-called reset adjustment whereby on a change in an operating condition, such, for example, as a change in the load on a furnace which is being controlled to minimize variations in the furnace temperature, the tendency of the furnace load or other operating condition change to increase or decrease the furnace temperature or other controlling condition may be neutralized.

The preferred form of the invention also ineludes means for effectin an adjustment in the rate of reset adjustment whereby on a change in furnace load, or other analogous operating condition, the compensating adjustment necessary to the maintenance of the approximately constant value of the furnace temperature or other controlling quantity may be varied to the end that the time required for full compensation for the operating condition change may be reduced to the practical minimum possible without risk 01' objectionable hunting.

In the preferred form of the invention provisions are also included for readily effecting a socalled throttling range acflustment whereby the extent to which the fuel valve or the regulator is adjusted in response to a given change in the furnace temperature or other control quantity may be varied.

The various features of novelty which characterize my invention are pointed out with particularity inv the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages, and specific objects attained with its 1158. reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described preferred embodiments of the invention.

Of the drawings:

Fig. 1 is a. control network diagram illustrating one form of the invention;

Fig. 1A illustrates a modification of a portion of the arrangement of Fig. 1;

Fig. 2 illustrates a modification of the arrangement of Fig. 1 in which physically stationary reset means are employed;

Fig. 3 illustrates another modification of the arrangement of Fig. 1 in which physically stationary means are employed to effect the desired reset and initially large corrective adjustments:

Figs. 4-9 illustrate further modifications oi the arrangement of Fig. 1 in which the various physical adjustments of Fig. l are effected by physically stationary means;

Fig. is an elevation of a control panel on which a control instrument and associated control devices are mounted;

Fig. 11 is an end elevation of the instrument mechanism of Fig. 10; and

Fig. 12 is a perspective view of automatic switch adjusting means included in the instrument shown in Figs. 10 and 11.

In the embodiment of the invention, illustrated more or less diagrammatically in Fig. 1, the fuel supply to a furnace i is regulated by the adjustment of a fuel supply valve 2 in automatic response to variations in the furnace temperature a required to maintain that temperature approximately constant. The furnace temperature is measured by means shown diagrammatically as comprising the bulb 3 of a fluid pressure type thermometer connected to a measuring instrument which includes an actuating element in the form of an are shaped Bourdon tube 4 to which the thermometer bulb pressure is transmitted by flexible tubing 5. On an increase or decrease in the furnace temperature, the resulting fiexure of the Bourdon tube 4 moves a contact 8 along a measuring instrument slidewire resistor I up or down as seen in Fig. 1. As shown, the contact 8 is mechanically connected to the Bourdon tube 4 by a strap 6a or by any other suitable means. The initial effect of any movement of the contact 6 is to unbalance a normally balanced electrical control network 8 including the resistor 1 and thereby to energize a reversible electrical control motor 9 for operation in the direction to effect a corrective adjustment of the fuel valve 2 and a corresponding network rebalancing adjustment of a contact ill. The motor 9 is a two-phase rotating field alternating current motor and operates in one direction or the other accordingly as alternating current of one phase or of opposite phase is applied to one of its windings. The shaft of the motor armature is operatively connected to the movable element of the fuel valve 2 and to the contact ID, the latter of which, as shown, is adjustable along a slide-wire resistor II included in the electrical control network 8.

As illustrated, the electrical control network 8 is made up of two sections i2 and I! each of which comprises a Wheatstone bridge network. Section I2 is energized by the connection of its energizing terminals by conductors I4 and I5 to the secondary winding l8 of a transformer I! having another secondary winding l8 and a line voltage primary winding i9 connected to alternating current supply conductors L and L An adjustable resistance 20-is inserted in the conductor It as shown to permit adjustments of the voltage supplied the bridge network [2. Two opposed arms of the bridge network l2 include the slidewire resistance 1 and the remaining opposed arms are made up of a slidewire resistance 2l along which a contact 22 is adapted to be adjusted.

Similarly, the bridge network I3 is energized by the transformer secondary winding l8- and has its energizing terminals connected thereto by conductors 23 and 24, in the latter of which an adjustable resistance 25 i inserted. The bridge network i3 includes the slidewire resistance II in two opposed arms and a slidewire resistance 26, along which a contact 21 is adjustable, in its remaining arms. The contacts 22 and 21 are electrically connected together and are carried at the opposite ends of a rotatable arm 28. The arm 28 is mounted at a point intermediate its ends and is adjusted by a reversible electrical motor 29, the operating circuits for which are described hereinafter, and, on rotation of the motor 29, moves the contacts 22 and 2! in opposite directions along their respective slidewires.

As illustrated, unbalance of the electrical control network 8 is adapted to be detected by an electronic valve 30 which may be a trlode including an anode, a cathode, and a control electrode and comprises a part of an amplifier 33 which may be of any well known type. The contact 6 of the control network 8 is connected by a conductor 3| to the cathode of valve 30 and the control electrode of the latter is connected by a conductor 32 to the contact ll! of said network. The valve 30 controls the output current of the amplifier 33 in accordance with the unbalance of the control network 8 so that on unbalance of said network produced by movement of the contact 6 in an upward direction along slidewire 1, the alternating current in the output circuit of the amplifier will be of one phase, and on unbalance of the network 8 produced by downward movement of the contact 6, the said output current will be of opposite phase. 33 are connected by conductors 34 and 35 to one phase winding 36 of the motor 9, the other phase winding 31 of which is connected through a suitable condenser 38 to the alternating cur- The output terminals of the amplifier rent supply conductors L and L The amplifier 33 is so connected and energized that on unbalance of the network 8 in one direction, the output current. thereof will be in phase with the voltage ofthe supply conductors and on unbalance of the network 8 in the opposite direction, the said output current will be displaced 180 with respect to the voltage of the supply conductors. Thus, on unbalance of the network 8 in one direction, the motor 9 will be energized for rotation in the direction to effect a corrective adjustmentof the fuel supply valve 2 and a corresponding network rebalancing adjustment of the contact In along the slidewire resistance ll. Such an automatic neutralization of an initial control adjustment is sometimes referred to as a follow-up action.

The apparatus illustrated in Fig. 1 includes means for effecting a control range or follow-up adjustment commonly referred to as a throttling; range adjustment particularly when the control element is a valve such as the valve 2. The throttling or control range adjustment varies the extent of adjustment of the valve 2 produced by a given movement of the contact 6 along the resistance I. The extent of the valve adjustment produced by a given change in the adjustment of the contact 6 depends upon the magnitude of the adjustment of the contact I along the resistance ll required to rebalance the control network 8 when the latter is unbalanced by a given change in position of the contact 6. In the form of the invention shown in Fig. l, the throttling range adjustment is effected by varying the amount of the resistance which is connected in the current supply conductor 24 to the bridge network l3. The resistance 25 may be manually adjusted by the rotation of a throttling adjustment knob 39.

As will be apparent the effect of an increase in the amount of resistance 25 in the conductor 24 is to decrease the voltage supplied the bridge network l3 and thereby to increase the extent of movement of the contact I!) required to rebalance the control network when the latter has been unbalanced by a departure of given magnitude of the contact 6. Similarly, the effect of a decrease in the amount of resistance 25 in the conductor 24 is to increase the voltage supplied the bridge network l3 and thereby to decrease the extent of movement of the contact I0 required to rebalance the control network when the latter has been unbalanced by an adjustment of given magnitude of the contact 6. The relation between the unbalancing movement of the contact 6 and the consequent rebalancing movement of the contact l0 may be kept linear for all throttling rang adjustments of the knob 39, and the sensitivity of the apparatus may be kept substantially constant over the entire range of contact movement.

The apparatus shown in Fig. 1 also includes means for automatically effecting resetting ad justments and thereby compensating for a change in furnace load or other condition which tends to vary the value of the controlling condition which the apparatus tends to maintain. In general, such reset adjustments must be effected slowly if hunting is to be avoided in any control system in which the effect of a change in the control force on the value of the control quantity is delayed, is it may be delayed in furnace control by temperature lag. The latter is dependent upon the heat storage capacity of the furnace, and affects the rate of response of the furnace temperature to a change in the rate of furnace heat supply. A control system effect analogous to that due to temperature lag is produced in different ways, for example, by the inertia of movable machine elements in controlling the operation of machinery, and by the fluid storage capacity of portions of a fluid distribution system in which fluid pressure or rates of flow are controlled.

In the form of the invention illustrated in Fig. 1, such resetting adjustments are effected by the adjustment of the contacts 22 and 21 along their associated resistances 2| and 26 by the reversible motor 29 and may be continuously effected as long as the contact 6 is deflected from the position corresponding to the desired value of the condition. If desired, the resetting adjustments may be effected only when the contact 6 is defleeting from its normal position and eliminated when the contact 6 is being returned to that position. Or, if desired, the resetting adjustments may be effected as long as the contact 6 is deflecting from its normal position or is stationary in a deflected position and eliminated when the contact 6 is being returned to that position. An important advantage in producing the desired reset adjustments when the contact 6 is deflecting or is deflected from its normal position and eliminating those adjustments when the contact 6 is being returned to its normal position, is the elimination of the tendency to hunting which is encountered in certain applications when the reset adjustments are continuously effected as long as the contact 6 is deflected from its normal position.

The reset motor 29 is energized for operation in one direction or the other by the energization of one or the other of its field windings 43 and 4| each of which has one terminal connected to the alternating current supply conductor L The second terminal of the motor winding 46 is connected by a conductor 42 to a stationary elongated contact 43 and the second terminal of the motor winding 41 is connected by a conductor 44 to a stationary elongated contact 45. The contacts 43 and 45 are electrically insulated from each other and are arranged end to end along different portions of the path of movement of a contact 46. The contacts 43 and 45 have a similar elongated contact 41 and 48, respectively. arranged in parallel therewith. The contact 46 is adjusted by the Bourdon tube 4 in correspond ence with the adjustments of the contact 6 and is adapted to electrically connect the contact 43- r to the contact 41 when the contact 6 is displaced in the downward direction from the position corresponding to the desired value of the condition. Similarly, the contact 46 is adapted to connect the contact 45 to the contact 48 when the contact 6 is displaced in the upward direction from that position.

The contacts 41 and 48 are connected to the center poles of a double pole double throw switch S and when the switch is thrown to the right, as shown, the contacts 4'7 and 43 are connected together and to the alternating current supply conductor L In this position of the switch S. one or the other of the windings 40 or 4| of motor 29, depending upon the direction of deflection of contact arm 46, will be continuously energized as long as contact arm 46 is deflected from its normal position and electrically connects contacts 43 and 41 or contacts 45 and 48. The resulting rotation of the motor 29 will be in the direction to create a further unbalance in the network 8 and hence a further rebalancing adjustment of the contact I!) and an additional adjustment of the valve 2 in the corresponding direction. The additional adjustment of the valve 2 is that required to effect a return of the furnace temperature to the desired value and thereby the adjustment of the contact 46 to a position intermediate the elongated contacts 43, 4'! and 45, 48 in which position the motor 29 is deenergized.

When it is desired to effect the resetting adjustments of the system only when the contact is being adjusted away from its normal position and to eliminate the resetting adjustments when the contact 6 is being returned to that position, the switch S is adjusted to the left, as seen in Fig. 1. In this position of the switch S it will be noted the contact 41 is connected by a conductor 59 to a stationary contact 50 and the contact if; is connected by a conductor to a stationary contact 52. The contacts 50 and 52 are arranged on opposite sides of a deflecting member 53, which normally assumes a position intermediate the contacts 50 and 52 but deflects in one direction or the other to engage one or the other of the contacts accordingly as the current in the output circuit of the amplifier 33 is of one phase or of opposite phase. To this end the deflecting member 53 may desirably be car .ried by the deflecting coil or winding 54 of a dynamometer 55. The winding 54 of the dynamometer 55 is connected by conductors 56 and 51 to the output terminals of the amplifier 33' and, as shown, is connected in parallel to the winding of motor 9. The dynamometer 55 also includes a stationary winding 58 which is connected through a suitable condenser 59 to the alternating current supply conductors L and L so that when the amplifier output current is of one phase the member 53 will be deflected into engagement with the contact 50 and when the said output current is of opposite phase the member 53 will be deflected into engagement with the contact 52. On such deflection of the member 53, an energizing circuit will be closed to one or the other of windings or 4| of the motor 29 if at that time the contact 46 is deflected from the normal desired position in a direction corresponding to the direction of de fiection of the member 53.

To produce the resetting operation when the contact 6 is moving away from the desired position, the dynarnometer 55 is so conformed that the member 53 will be deflected into engagement with the contact 52 when the contact 6 is being moved in an upward direction. At that time the contact will bridge the elongated contacts 45 and 48 and consequently energization of the winding ill of motor 29 will be eifected. The resulting rotation of the motor 29 will be in the direction to create a further unbalance in the network 8 and hence a further rebalancing upward movement of the contact In and an additional closing adjustment of the valve 2. The additional closing adjustment of the valve 2 effected is that required to effect a return of the furnace temperature to the desired value and thereby the adjustment of the contact 6 to a position corresponding to that desired value. On such return movement of the contact 6 to the desired position, it is noted the reset motor 25 is deenergized since the member 53 of the dynamometer will then be deflected into engagement with the contact 50. Accordingly, on such return adjustment of the contact 6, the

valve 2 will be given an opening movement of extent corresponding only to the follow-up movement of the contact It required to rebalance the network 8 before the resetting operation was eirected and therefore less the closing movement by an amount corresponding to the amount of reset.

If it is desired to effect the resetting operation when the contact 6 is moving toward the normal desired position, as may be desirable in a slowly cooling and rapidly heating process, the dynamometer 55 is so conformed that when the contact arms 6 and 45 are being adjusted away from their normal positions, the reset motor 29 will be inactive, and accordingly the valve 2 will initially be given a closing adjustment, on an increase in temperature, of an amount required to balance the network 3. On return of the furnace temperature to the desired value, however, the reset motor 29 will be energized for rotation in the direction tending to reduce the unbalance of network 8 and hence a smaller adjustment of contact l0 along resistance I I will be sufficient to rebalance the network. Thus, in this case also, the valve 2 will be opened an amount less than the closing adjustment by an amount corresponding to the extent of reset.

As will be understood, the speed characteristic of the reset motor 29 and the ratio of reset motor motion to motion of the contacts 22 and 2'! should be chosen so as to be suitable for an average or normal full compensating period which varies with the character of the operation controlled. For example, it may be five minutes with one furnace and twenty minutes with another. In general, the operation of the valve motor 9 directly due to adjustment of the contact 6 will be effected much more quickly than that directly due to the adjustment of contacts 22 and 22.

In the form of the apparatus thus far described wherein the resetting adjustments are eiiected only when the contact 6 is departing from its normal position, it is possible for the system to stabilize with the contact 6 at a position displaced from the desired position. In order to guard against this contingency the contacts 50 and 52 may be normally held into engagement with the deflecting arm 53 of the dynamometer 55 by spring or other means as illustrated in Fig. 1A so that whenever the contact arms 6 and 45 stabilize away from their normal positions the reset motor 29 will be energized. When the contact arms 6 and 48 are departing from or returning to their normal positions, the arm 53 will move out of engagement with one of the contacts 50 or 52 and deflect the other contact against the opposing force of the spring means carrying that contact. Thus, it will be apparent, that with the modification illustrated in Fig. 1A the resetting adjustments of the system may be eliminated when the contact 6 is either departing from or moving towards its desired position and efiected during the remainder of the time in which the contact 6 is deflected from its normal position.

An alternative arrangement has been illustrated in Fig. 1 for preventing the stabilization of the system with the contact 6 displaced from the desired position wherein provisions have been made to periodically close an auxiliary circuit to the reset motor 29, which circuit will be effective whenever the contact 45 is displaced from the desired position intermediate the elongated contacts 43, 41 and 45, 48. To this end th center asiavii tap of a suitable resistance 60, which has its terminals connected to a respective contact I and 52 of the dynamometer 55, is connected by a conductor ii to the supply conductor L A switch 82 adapted to be alternately moved to its open and closed positions is inserted in the conductor BI and operates to alternately energize and deenergize the auxiliary circuit to the motor I. referred to above. As shown, a cam 83 driven by a continuously rotating unidirectional motor 64 and in cooperative relation with the switch 82 is provided to alternately and periodically actuate the latter to its open and closed positions to effect such operation. With this arrangement, as in the modification shown in Fig. 1A, it will be apparent the system can never stabilize until the contact 48 has returned to the desired position intermediate the elongated contacts. l8, l1 and 45, ll.

The apparatus illustrated in Fig. 1 also includes provisions for effecting an initially large corrective adjustment in the setting of the fuel valve 2 on a change in the furnace temperature from the desired value. This initially large corrective adjustment is removed before the furnace temperature has returned to the desired value to the end that the tendency to hunting which would otherwise result from such large corrective adjustment is eliminated.

In the form of the invention illustrated in Fig. 1 such initial over-adjustment of the fuel valve 2 is effected by varying the amount of resistance 20 which is connected in the electrical current supply conductor ii to the bridge network II. The resistance 20 may be. manually adjusted by the rotation of a knob 65' and is adapted to be shunted by a switch 66, to be described, while the contact 6 is moving away from the position corresponding to the normal desired temperature. The shunt connection about resistance 20 is removed as soon as the contact i begins to return to that position.

As illustrated, the switch 88 comprises an elongated member 81 which is suspended by spring means 68 and 89 adjacent and parallel to the slidewire resistance 1. Such suspension provisions are made to permit longitudinal adjust-- ment of the member 61 in one direction or the other relatively to the slidewire resistance I on deflection of the contact 6 from its desired position. To this end a shoe ll carried by the Bourdon tube 4 is arranged in frictional engagement with a surface of the member 61 so that on expansion or contraction of the tube l the member 61 will be adjusted against the opposing action of the spring means It and 60 in one directionor the other. The friction exerted by shoe 10 on member 81 i so adjusted as to maintain the member 81 in its defiectedposition against the opposing action of the spring means 68 and 69 when the tube 4 is not expanding or contracting.

Electrical contacting arms II and 12 are mounted on opposite ends of the member 81 and are insulated from each other. The contacting arms II and II are arranged in cooperative relation with a respective stationary contact I3 and 1|, which are electrically connected by a conductor I5, and are out of engagement with said stationary contact; when the member I1 is in its undeilected position. On movement of the member 81 is a downward direction, as seen in Fig. i. the contacting arm II will engage the contact 13 and on movement of the member I! in an upward direction the contacting arm 12 will engage the contact ll.

As shown, the contacts II and I! are electrically connected to a respective one of a pair of elongated contacts I6 and 11 which are arranged end to end adjacent and parallel to the slidewire resistance 1. The adjacent ends of contacts I8 and 11 are separated from each other by a suitably mall distance, preferably slightly ,more than the width of a contact ll.- Contact I! is carried by the Bourdon tube 4 and is adapted to engage one or the other of thecontacts l6 and 11 as the tube 4 expands and contracts. The contact 18 is connected by a conductor 1! to one terminal of the resistance 20, and the conductor 16 which joins the contacts I3 and I4 is connected by a conductor '0 to the other terminal of the resistance 20.

When the contact I! is displaced in the upward direction from the position intermediate the contacts It and If and the member i1 is also displaced in the same direction'from its undeiiected position, a low.resis'tance shunt circuit will be established about the resistance 20. This shunt circuit may be traced from one terminal of the resistance 20 to conductor 1!, contact Ii, contact 11, contact arm 12, cdntact I4, conductor 1',

and conductor II to the other terminal of the resistance. Similarly, when the contact II and the member '1 are displaced downwardly from their undeflected positions, a low'resistance shunt will be closed about the resistance II from one terminal of resistance 20 to conductor 1!, contact ll, contact ll, contact arm ll, contact 13, and conductors I! and I! to the other: terminal of the resistance. It will be noted that when the member 01 and contact 18 are.deflected in opposite directions from their normal positions, as will,

be thecase when the/furnace temperature is returning to the desired value, the shunt circuit about the resistance '20 cannot be closed. For the condition when one or the other of the elements II or "is in its undeflected position and the other element is deflected from that position, as may possibly be the case when the furnace temperature is at the desired value, the shunt circuit about the resistance 20 cannot be closed.

The effect of closing the shunt circuit described about resistance 2! is to increase the voltage supplied the bridge network I: and thereby to increase the extent of movement of the contact ll along resistance H required to rebalance the control network I. The extent, to which the unbalance of the network 8 is so increased is dependent upon the adiustment of the knob 85.

Thus, with the arrangement described, as long a; j

the contact 8 is moving away from the position corresponding to the desired furnace temperature value and has not started back to that posi tion. the network 8 will be unbalanced to a greater extent than it would be by the movement of the contact 6 alone, and accordingly the valve 2 will initially be adjusted a corresponding additional amount.

The eifect of such initial magnified corrective adjustment of the valve 2 is to make the new rate of fuel supply to the furnaceir'significantly higher or lower than the new rate of heat output, accordingly as the furnace temperature is lower or higher than the desired value. Subsequently, when the furnace temperature begins to return to the desired value, the magnification of the correction provided is reduced and hence any tendency to overshooting ofthe desired temperature value is avoided. with this apparatus,

therefore, a large initial effect, calculated to arrest and reverse the trend of the condition, is obtained which is reduced in response to the subsequent trend of the condition and is combined with the follow-up and resetting actions previously described.

In some cases it may be desirable or necessary to effect the reduction of the magnification of the corrective adjustment of the valve 2 at an earlier point in the furnace temperature cycle of varia tion in order to effect suitably fast restoration of the furnace temperature to the desired value while avoiding any tendency to hunting. Such operation may be obtained with the apparatus of Fig. 1 with slight modification, namely, suitable reduction in the degree of friction between the friction shoe l and the member 81 relatively to the tension and compression strength of the spring means 88 and 88. For example, the degree of friction between the shoe I0 and the member 81 may be adjusted by applying a suitable viscous lubricant to the surface of member 81. With such a modified arrangement, when the contact 8 is moving faster than a predetermined minimum rate away from its desired position the member 81 will be deflected in the same direction in which the contact 8 is moving, but when the movement of contact 8 falls below that minimum rate the equalizing action of the springs 88 and 68 will return the member 81 to its undeflected position and thereby effect a reduction in the voltage applied the bridge I! by opening the shunt circuit about the resistance 20. Thus, the magnification in the corrective adjustment of valve 2 effected as a result of shunting of resistance 20 will be removed as soon as the rate of movement of the contact 8 away from its desired position falls below a predetermined minimum value.

With the apparatus shown in Fig. 1 operating with furnace heat output and input exactly equal with the furnace temperature at its normal value, if there should be a particular change, for example, an increase in the furnace heat output, followed by a prolonged period of operation in which no further change in the rate of heat output occurs, it will be apparent the operating eflect of the control apparatus described will be such as to produce the following results: an initial adjustment of the valve 2 which increases the fuel supply to make the new rate of furnace heat supply significantly higher than the new rate of heat output; a subsequent control effect tending to suddenly reduce the rate of heat input; and as the furnace temperature returns to the desired value a compensating adjustment in the rate of heat input which tends to neutralize or cancel out a portion or all of the remaining portion of the initial adjustment of the valve 2 depending upon the magnitude of the reset adjustment required to maintain the furnace temperature at the desired value with the new rate of heat output.

For the particular operating conditions described above it may be assumed that the ideal calibration of the control apparatus would be that which would return the furnace temperature to its normal value in the shortest possible time. In practice, however, the control quantity, namely, the furnace temperature in the apparatus shown in Fig. 1, is subject to fluctuation as a result of rapid change in furnace load or some other operating condition so that a new adjustment of the valve 2 is apt to occur before the follow-up and compensating adjustments resulting from a previous change in the control quantity have been completed. In practice, therefore, the adjustment of the various circuit constants may not and usually will not be precisely those which would be ideal for any one assumed particular instantaneous change in the control quantity. The apparatus shown in Fig. 1 permits by suitable adjustment of the resistances 20 and 25 of a wide range of variation in the control system characteristics thereby permitting a desirably good regulation under very different conditions of control operation.

In Fig. 2 I have illustrated, more or less diagrammatically, a modification of the arrangement of Fig. 1 which includes physically stationary means for controlling the resetting adjustments of the valve 2 to compensate for changes in the furnace load or other operating conditions which tend to vary the value of the quantityit is desired to maintain constant.

In the control system illustrated in Fig. 2, an electrical control network 8| is employed in lieu of the control network 8 of Fig. 1 and the resetting provisions of Fig. 1 have been replaced by physically stationary means included in the network 8i and adapted to produce the same result. Thecontrol network 8| is made up of two sections each of which comprises a Wheatstone bridge indicated by the reference numerals 82 and 88. The Wheatstone bridges 82 and 83 are energized by a respective battery 84 and 85. A resistance 86 is connected in the energizing connection to the bridge network 82 and a resistance 81 is connected in the energizing connection to the bridge network 83. The resistance 88 is adjustable, as, for example, by a knob 88 and is provided for effecting the desired initial magnification of the corrective adjustment of the fuel valve 2. The resistance 88 is adapted to be short circuited by means of a switch 88 which may be identical to and actuated in the same manner as the correspondingly identified part of Fig. 1, but has been only shown schematically in Fig. 2 in order not to confuse the drawing. The resistance 81 is adjustable by a knob 88 and is provided to permit throttling range adjustments of the network 8|.

As illustrated, the bridge network 82 includes the slidewire resistance I in two of its opposed arms and along which the contact 8 carried by the Bourdon tube 4 is adapted to move. The remaining aims of the network 82 are made up of fixed resistances and 8|. The bridge network 83 includes the slidewire resistance along which the contact i8 is adapted to move, in two of its opposed arms and fixed resistance; 82 and 83 in its remaining arms. The point of engagement of the resistances 88 and 8| is connected by a conductor 84 in which a condenser 88 is inserted to the point of engagement of the resistances 92 and 83. The point of engagement of the resistances 80 and 8| i also connected to the contact H! of the network 88 by a conductor 88 in which a high resistance 81 is inserted. The resistance 81 may be adjustable by a knob 88.

In this form of my invention unbalance of the electrical control network 8| is adapted to be detected by a pair of electronic valves 88 and I00, both of which may comprise triodes having an anode, a cathode and a control electrode. The cathodes of the valves 88 and I08 are connected together and through a biasing resistance H to one input terminal of a uitable amplifier 88a and to the contact 8 of the bridge network 82. The anodes of the valves 88 and I88 are connected to the remaining input terminals of the amplifier 33a. The control electrode of the valve 99 is connected directly to the negative terminal of the biasing resistance IN and the control electrode of the valve I99 is connected to that terminal through the control network 9|. With this arrangement it will thus be apparent the 'poten tial of the control electrode of the valve I99 is adapted to be varied in accordance with the unbalance of the control network 9|, and that the potential of the control electrode of valve 99 is adapted to be maintained relatively constant.

The amplifier 33a may be of .any well known type and may be identical to the amplifier 39adescribed hereinafter in connection with Fig. 4 and is adapted to supply alternating current, in phase with or displaced 180 in phase with respect to the voltage of the supply detectors L and L to the winding 39 of motor 9 accordingly as the conductivity of the valve I99 rises above or falls below the conductivity of the valve 99.

When the control network 9| is in a balanced condition, the conductivities of the trlodes 99 and I99 will be approximately equal and for that condition the output current of the amplifier 33a will be zero.

In Fig. 2, as in the arrangement of Fig. 1, the initial effect of a furnace temperature change is to unbalance the control network 9| so as to create a difference in potential between the contacts 6 and I9. In Fig. 2, however, this difference in potential is unidirectional since direct current is employed to energize the networks 92 and 99.

-When the polarities of the batteries are that shown, an increase in furnace temperature and a subsequent upward adjustment of the contact 5 effects an increase of the potential of the contact 5 in the positive direction with respect to the potential of the contact I 9. This will increase the potential of the control electrode of the valve I99 in the negative direction to thereby reduce the conductivity of that valve and effect the supply of alternating current to the motor winding 36 of the correct phase to produce operation of the motor 9 in the direction to give an upward rebalancing adjustment to the contact I9 and a closing adjustment to the fuel valve 2. Similarly, a decrease in furnace temperature and a subsequent downward adjustment of the con tact 6 will operate to increase the conductivity of the valve I99 with respect to that of the valve 99 to thereby produce energization of the motor 9 for rotation in the direction to effect a downward rebalancing movement of the contact I9 and an opening adjustment of the valve 2.

In this embodiment of my invention, the rebalancing adjustments of the contact I9, produced as described above, are not continuous in their effect when the contact 5 is displaced from its normal position, but gradually diminish, and if no further adjustments were made, would disappear entirely. This phenomenon occurs as a result of the connection of the condenser 95 and resistance 91 as shown and is employed as described hereinafter to effect the desired resetting adjustments of the control ap aratus of Fig. 2.

With the furnace operating condition such that the control network 9| has been balanced for an appreciable length of time'with the contacts 9 and I9 at intermediate points along their respective slidewires, it will be seen that the difference in potential between the contact- 6 and the bridge point I92 will be zero. The difference in potential between the point of engagement of the bridge resistances 92 and 93, indicated by the reference numeral I93, and the contact I9 will also be zero. Therefore, there will then be no charge on the condenser 95 and since the difference in potential between the contact I9 and the bridge point I93 is zero, the current flow through resistance 91 will be zero whereby the potential drop across that resistance will be zero.

On a change in the furnace heat output, for example, on an increase in the heat output followed by a prolonged period in which no further change in the rate of heat output occurs, the fur-v nace temperature will decrease and subsequently the Bourdon tube 4 will effect a downward adjustment of contact 6 along resistance I. Such adjustment of the contact 5 will effect a reduction in the negative potential applied to the control electrode of valve I99 and thereby produce energization of the motor for operation in the direction to give a rebalancing adjustment of the contact. I9 in a downward direction and an opening adjustment of the valve 2, That rebalancing adjustment of the contact I9 i not permanent in-its effect, as noted hereinbefore, because of the flow of charging current to the condenser 95 through the high resistance 91 produced by the establishment of a difference in potential between the contact I9 and the bridge junction I93. This charging current gradually produces a potential across the condenser 95.

which potential is in opposition to that between the contact I9 and the junction I93, and thereby graduallyreduces the effectiveness of the rebalancing adjustment of the contact I9.

As the rebalancing effectiveness of the ad-- The motor 9 and thereby the contact I9 will therefore creep along in the direction of the initial adjustment as long as this operative condition obtains to tend to maintain the balance of the network 8| and to give an additional opening adjustment to the valve 2.

It will thus be noted the establishment of a potential on the condenser produces the precise effect produced by movement of the contacts 22 and 21 along their respective slidewire resistance 2| and 29 in the Fig. 1 arrangement. In Fig. 2 however, as noted above the charging of condenser 95 is continuous as long as contact 5 is displaced from its desired position and,

hence, the rebalancing adjustment of the contact I9 is continuous until the contact 5 has returned to that position.

The rate at which such a potential is built up on the condenser 95 is determined by several factors including the extent of departure of the contact 9 from its desired position and thereby of the contact I9, the capacity of the condenser 95, and the magnitude of resistance 91. In accordance with the present inventiomthat rate is desirably slow so that on an initial adjustment of the contact 5, the total effect of the charging of the condenser 95 during the time of displacement of the contact 5 from its desired position is only such as to compensate for the initial furnace characteristic change which caused the reduction in temperature.

When the contact 5 begins to return to its normal position, the network 9| will be unbalanced in the opposite direction, and the motor 9 wat the desired value.

will consequently be energized for rotation in the opposite direction to move the contact it back towards its initial position and to effect a closing adjustment of the valve 2. It is noted that during such return adjustment of the contact 5, the condenser 95 continues to charge in the same direction as before. Thus, when the contact 6 reaches its normal position, a potential will have been built up on the condenser 95 which will be of a magnitude determined by the extent and duration of deflection of the contact 6 from its normal position. As will be apparent, in order for the control network 8| to assume a stable condition for the new furnace conditions, that potential on the condenser 95 must be compensated for or balanced out. This end is obtained in this form of my invention, as in the arrangement of Fig. l, by the contact ill coming to rest at a point short of its initial position; At this point the potential between the contact Ill and junction I03 will be exactly equal and opposite to the potential on the condenser 95.

The system of Fig. 2 will then stabilize, if no further change in the furnace operating conditions has occurred meanwhile, with fuel being supplied to the furnace at a new and higher rate determined by the extent and duration of departure of the contact 6 from its normal position. It will be apparent that for the case of decreased heat output of the furnace, the control apparatus of Fig. 2 will effect a reduction in the supply of heat to the furnace to thereby return and maintain the furnace temperature It is thus seen the adjustment effected by the motor 9 is a true compensating adjustment making it possible to maintain approximately the same furnace temperature notwithstanding changes in the furnace load, for example, as may result from substantial changes in amount of material heated in the furnace, and furthermore has temporarily provided an additional corrective effect in the supply of heat to the furnace to thereby quickly return the furnace temperature to the desired value. In this connection it is noted the actuation of switch 68, as described hereinbefore, produces a magnification in the corrective effect in the heat supply during the initial stages of the furnace temperature departure with the result that the furnace temperature is returned in the shortest time possible to its desired value.

In the governing or control system of the general character shown in Fig. 2, the rate at which compensating adjustments are effected must be suitably related to the constants, and particularly the time lag constant of the apparatus or process controlled, if hunting is to be avoided. In the arrangement of Fig. 2 the rate at which the fuel valve 2 is adjusted while the contact 8 is displaced from its normal position may be controlled in a number of different ways, for example, by adjustments of the resistance 97, the bridge resistance values, the relative currents drawn from the batteries 84 and 85, and the capacity of condenser 95. As one such means, I have illustrated the resistance 91 as adjustable by a knob 88, the effect of adjustment of which is to vary the charging rate of condenser 95 and thereby the speed of motor 9 on a given deflection of contact 6 from its normal position.

It is noted that, with the control apparatus of Fig. 2, if the furnace is manually shut down for short periods by the closing of additional fuel supply valve means not shown, as for example, in

a periodic process, the cooling of the furnace will produce a downward adjustment of the contact 6 and if the control network II is maintained energized during the periods of shut down, a followup adjustment of the contact in in a downward direction will be effected. Thereafter a charge will begin to be established on the condenser 95' and as noted hereinbefore this charge will tend to produce a further downward adjustment of the contact I0 if the latter is not already in its extreme low position. The potential on the condenser 95 will continue to build up even after the contact III has reached its extreme low position until the condenser potential has become equal in magnitude to the potential between the contact l0 and the bridge junction I03. If the time of furnace shut down is short and the rate of furnace temperature rise suitably fast, the magnitude of the charge on condenser 95 and thereby the extent of adjustment of the contact l0 and valve 2 will not be significant, but if the time of shut down is appreciable or the rate of furnace temperature rise is slow, the charge on the condenser 95 may become equal and opposite to the potential between the contact l0 and the bridge junction I 03 even though the contact I0 is in its extreme low position. Furthermore, even if the network 8| is deenergized while the furnace is shut down, and energized when the furnace is started up, if the rate of furnace temperature rise is very slow, the potential on condenser 95 may build up to a value which is equal and opposite to the potential between the contact I0 and the bridge junction Ill! before the contact Ill begins to move upward.

In either of the cases above mentioned the effect of the establishment of such a potential on condenser 95 is to delay the follow-up adjustment of the contact iii in anupward direction and thereby the closing adjustment of valve 2 until the contact 6 has reached the position corresponding to the normal desired" temperature. Thus, although the furnace temperature is rising, the control apparatus of Fig. 2 will be insensitive to that rise until the'contact 8 has reached and passed its normal, desired position.

The effect of such insensitivity of the control apparatus is to permit he supply of more fuel to the furnace than is needed to bring the temperature to the desired value and hence to produce overshooting and objectionable hunting.

One means of avoiding this condition is to leave the control network 8! deenergized until the contact 8 has nearly reached its desired position.

In Fig. 3 I have illustrated, more or less diagrammatically. a modification of the arrangement of Fig. 2 embodying provisions for eliminating the undesirable insensitivity of that arrangement as described, and hence, for minimizing the tendency to hunting when first heating up the furnace. In Fig. 3 also means alternative to the means shown in Fig. 2 for energizing the-bridge networks 82 and 83 and other features to be de-.v

scribed have been illustrated.

As illustrated, energizing current is supplied th; Wheatstone bridge networks 82 and I! of the arrangement of Fig. 3 from the secondary windings I04 and I05, respectively of a transspectively,

the energizing connection to the bridge network 83. Thus, in this arrangement, the bridge networks are energized by pulsating direct current. If desired, suitable filters may also be employed to produce steady direct currents for energizing the bridge networks. A resistance H and a resistance IIOa which are simultaneously adjustable in opposite directions, as for example, by a knob I I I, are connected in the energizing connections to the bridge networks 92 and 03, re-

and are provided for eil'ecting throttling range adjustments of the system.

The provisions referred to hereinbefore for eliminating the undesirable insensitivity of the control apparatus of Fig. 2 when the contact 6 has been maintained in its lowest temperature position for an appreciable period include means for preventing the establishment or maintenance of a significant potential on the condenser 95 whenever the contact 6 is in that position. The means referred to above include a contact H2 arranged in the path of the movement of the contact 6 and positioned at a suitably low temperature position of the latter. The contact H2 is connected by a conductor I I3 to a junction I I4 at the lower end of resistance 93, as seen in Fig. 3. g

The effect of engagement of contacts 6 and H2 is to create a shunt about the bridge network 02 and the condenser 9i and thereby to eliminate the unbalance effects of the latter, and to establish a circuit to discharge the condenser 95 to thereby prevent the establishment of a potential of significant value thereon. When the unbalance effects of the bridge network 92 and condenser 95 are so eliminated, the contact I0 will come to rest at a position along the slidewire II which is at the same potential as the contact Ill. Accordingly, the position of the contact H4 is preferably so chosen that the valve 2 will then be in its fully opened position.

As will be apparent, the effect of preventing the establishment or maintenance of a potential of significant value on the condenser 95 while the contact 6 is in engagement with the contact H2 is to permit immediate upward adjustment of the contact I0 and thereby adjustment or the valve 2 towards its closed position on movement of the contact 6 towards its normal, desired position. Thus, it will be apparent the sensitivity of the control apparatus of Fig. 3 will not be impaired even it the contact 0 has been maintained in engagement with the contact H2 and the network 9| has remained energized for an unusually long period of time, and hence the tendency to objectionable hunting after such an occurrence is eliminated.

In addition to the provisions described above, I have illustrated in Fig. 3 an arrangement alternative to that shown in Figs. 1 and 2 for effecting an initial magnification of the corrective adjustment of the fuel valve 2, on a departure of the contact 0 from its normal position, tt i eturn the furnace temperature to the desired value in the shortest possible time while minimizing the tendency to hunting.

The initial magnification of the corrective adjustment of the fuel valve 2 produced by the apparatus of Fig. 3 is obtained by temporarily reducing the eflectiveness of rebalancing adjustments of the contact I0 along the slidewire resistance II whereby a magnified adjustment of the contact I0 and the valve 2 is effected. This temporary reduction in the eflectlveness of rebalancing adjustments of the contact I0 is pro- Hi and Ilia and condenser H8 is connected in shunt to the resistance 91. The resistance IIi may be varied in magnitude by adjustment of a knob Ill, and the resistance Ilia has associated therewith a contact IIib which is adjustable along the length thereof by a knob Illa. One terminal of the parallel connected resistances I I5 and Ilia is connected to the terminal of resistance 9'! which is connected to the contact I0 and the other terminal of that parallel resistance connection is connected by the switch Gin and the condenser I I6 to the other terminal of resistance 91. The contact Ilib is connected by the conductor 32 to the control electrode of the valve I00. It will be noted that the condenser Iii and a portion of resistance Ilia depending upon the adjustment of contact Ilib are connected between the junction I02 of bridge 82 and the control electrode of valve I00.

The connection of the condenser IIS and part of resistance Ilia between the bridge junction I02 and the control electrode of valve I00 tends initially to subdue changes in the potential between the junction I02 and the control electrode of valve I00 in response to adjustments of the contact I0. Accordingly, contact I0 will initially be given a magnified adjustment in eflecting an instantaneous rebalance of the network 8|. Specifically, when the switch 69a is in its closed position, only a fraction of the effect of the rebalancing adjustments of the contact I0 along its associated slidewire II will initially be effective to change the potential of the control electrode of valve I00, the magnitude or that fraction depending upon the adjustment of contact Ilib along resistance Ilia. As will be apparent, an adjustment of the contact I0 along the slidewire II will cause a fiow of charging current through the parallel resistances Hi and Ilia to the condenser I I6 to thereby build up a potential on that condenser which is of polarity opposite to that between the contact 6 and the bridge junction I02. That flow of charging current through resistance Ilia will produce a potential drop thereacross which is of the same polarity as the potential being established on condenser IIi. Since the condenser IIS and the portion of resistance Ilia to the right of contact Ilib are connected between the bridge junction I02 and the control electrode of valve I00, as noted hereinbefore, it will be apparent the network II will be stabilized at any instant when the sum of the potential on the condenser Hi and the potential drop across the portion of resistance Ilia referred to is exactly equal and opposite to the potential between the contact 6 and the bridge junction I02.

It is noted that during the initial stages of an adjustment of the contact I0 in response to an adjustment of the contact 6, the magnification in that adjustment will be determined principally by the position of contact Ilib along the resistance Ilia and will be proportional to the ratio of the total resistance of resistance Ilia to the portion of resistance 5 to the right of contact Ilib. Thereafter, a potential is gradually built up on condenser Hi and the charging current through resistance Ilia gradually decreases in value. The effect of reduction in the flow of charging current through resistance Se is the same as adjustment of the contact Ilib toward the left along resistance 5:; so that as the charging current gradually decreases and the potential on condenser IIB gradually increases the potential of the contact Il5b will gradually approach that of the contact I and those contacts will be at the same potential when the charging current flow through resistance I Ia is zero. The potential on the condenser IIG will then be exactly equal and opposite to that between the bridge junction I02 and the contact 6. As will be noted, the magnification in the adjustment of the contact I0 will have been reduced to unity during this process and accordingly the contact III will have been returned to its true follow-up position.

' Similarly, on a return adjustment of the contact 8 to its desired position the contact ID will immediately be given a magnified return adjustment to its initial position and thereafter the magnification in that adjustment will gradually be removed.

In the foregoing explanation of the operation of the apparatus of Fig. 3, the condition considered has been that wherein the contact 6 is given an adjustment away from its desired position and then remains stationary until returned to the desired position. It will be apparent, however, that in practice the adjustments of the contact 6 are ordinarily gradual. If the contact 6 is adjusted slowly and continuously in the same direction, the tendency for the magnification in the adjustment of the contact I0 to build up to its maximum value will be decreased by the potential which is permitted to build up on the condenser I I6 and accordingly the magnification will assume an intermediate value determined by the rate of adjustment of the contact 6. The magnification of the adjustment of the contact I0 will thus vary in proportion to the rate of adjustment of the contact 6 and will be larger when the rate of adjustment of the contact 6 is larger since the potential on the condenser IIG will not have time to build up to the same relative extent on rapid adjustments of the contact 6 that it will on slow adjustments of that contact.

The effective magnification in the adjustment of the contact l0 obtained is thus seen to be one which may be expressed mathematically in terms of rate of change since the magnitude of the magnification obtained is proportional to the rate of the change in the condition being controlled.

It is noted that when the speed of adjustment of the contact I 0 is suitably slow by virtue of suitable gearing between the motor 9 and the valve 2 and the contact II], the time of adjustment thereof may be relied upon to limit the initial magnified adjustments of the contact I0 to a desired value on a given adjustment of the contact 6, and accordingly, the resistance Ilia and contact II5b may be dispensed with and the control electrode of valve I00 connected to the right end terminal of the resistance II5 as shown in Fig. 4.

When the resistance Ilia and contact II5b are thus dispensed with, it is noted the initial tendency upon sudden changes in position of the contact 6 is to produce an adjustment of the contact l0 and the valve 2 to their extreme position, but when the maximum rate of adjustment of the contact I0 is suitably limited, as noted above, the maximum magnification may be kept within desirable limits without requiring the use of resistance ll5a and contact Il5b.

The resistances I I 5 and I I5a and condenser Ill are desirably so proportioned that the initial magnified adjustment of the fuel valve 2 is removed at a point near the ending of the time of increasing departure of the furnace temperature from the desired normal value and consequently the magnified adjustment may be made appreciable in magnitude to thereby effect an extremely fast return of the furnace temperature to the desired value. Furthermore, as explained hereinbefore, as the furnace temperature begins to return to the desired value, the contact II) will be given a. magnified adjustment towards its initial position and therefore the removal of the corrective control effect will be effected at a magnified rate.

The effect of such operation is to permit the initial corrective control effect in the supply of heat to the furnace to be much larger than would be possible otherwise without resulting in overshooting and consequent hunting. This end is obtained because the greater part of the corrective effect is applied during the time of increasing departure of the furnace temperature from the desired value and is removed at a suitable time before the furnace temperature has returned to the desired value to avoid overshooting and consequent hunting.

The duration and effective value of the magnification in the control effect produced by the condenser H6 and its associated elements, as described above, may be varied by adjustment of the knob Ill. The effect of adjustment of the knob Ill is to vary the time required to charge the condenser IIS to the potential between the contact 8 and the bridge junction I02.

In Fig. 4 I have further illustrated, more or less diagrammatically, a modification of the arrangement of Fig. 3 in which the rectifier I00 of Fig. 3 has been dispensed with so that alternating current is supplied the bridge network 82, and a plural stage condenser unit is employed for effecting the desired resetting operations. In Fig. 4 I have also illustrated in detail an amplifier 33a which may desirably be employed. It will be apparent the amplifier 33aof Fig. 4 may be employed in the arrangement of Fi s. 2 and 3, if desired.

The plural stage condenser arrangement referred to above for effecting the desired resetting adjustments includes a. condenser III and a. resistance IIB connected in series across the terminals of the condenser 85. As shown, the conductor 98, in which the resistance 91 is inserted, is connected to the junction of condenser I I0 and resistance II8. One effect of such a plural stage arrangement over the single stage form of Figs. 2 and 3 is to increase the time required to charge the condenser unit and hence to decrease the rate of adjustment of the resetting adjustments of the contact I0 and the fuel valve 2. It will be apparent more stages may be added, if desired.

The electronic amplifier 33a shown in detail in Fig. 4 includes a pair of electronic valves I20 and I2 I, which valves are heater type tetrodes including anode, cathode, filament, screen and control electrode elements, and the output circuits of which are coupled by means of a transformer I22 to the winding 35 of motor 9. As shown, the valves I20 and I2l may desirably be included in the same envelope. Anode voltage is supplied the valves I20 and I2I from the alternating current supply conductors L and U, and as illustrated, the supply conductor L is connected through a biasing resistance I23 shunted by a condenser I24 to the cathodes of the valves, which are connected together; and the supply conductor L is connected to a center tap on the secondary winding I25 of the transformer I22. One terminal of thetransformer winding I25 is connected to the anode of valve I20 and the other terminal of the winding is connected to the anode of the valve I2 I.

The conductivities of the valves I20 and HI are adapted to be controlled in accordance with the conductivities of the valves 99 and I00. To this end .the output circuits of the valves 99 and I are directly coupled to the input circuits of the valves I20 and I2I. As shown, the control electrode of the valve I20 is connected by a conductor I26 to the negative end of a resistance I29 in the anode circuit of the valve 99, and the control electrode of the valve I2I is connected by a conductor I21 to the negative end of a resistance I30 in the anode circuit of the valve I00. The positive ends of the resistances I29 and I30 are connected to the negative end of the biasing resistance I23 so that variations in the potentials across the resistances I29 and I30 will produce corresponding variations of the potentials on the control electrodes of the valves I20 and I2I.

Anode voltage is also supplied the valves 99 and I00 from the supply conductors L and L but it will be noted the valves 99 and I00 are connected thereacross in an inverse manner with respect to the connection of valves I20 and I2I. Thus, valves 99 and I00 and valves I20 and HI will be conductive only during alternate half 03'- cies of the supply line voltage and thereby the conductivities of the valves I20 and I2I will be controlled in accordance with the difference in the magnitudes of the currents conducted by the valves 99 and I00 during the preceding half cycle of the supply line voltage. As illustrated, a condenser I3I is connected between the anodes of the valves 99 and I00 for efiecting such control of a successive pair of valves during the next later half cycle.

With the arrangements of Figs. 2-4, it is noted that since the high resistance 91 is the only direct connection between the bridge networks 82 and 93, the said bridge networks must be well insulated from each other in order to guard against the establishment of a low resistance shunt about the resistance 91. The effect of such a low resistance in shunt to the high resistance 91 is to render the network 93 ineffective to rebalance the control network 9| on the occurrence of an adjustment of the contact 9 whereby the fuel valve 2 will be adjusted to its fully opened or closed position whenever the network 92 is unbalanced. It will be apparent, however, that by insulating the networks 92 and 93 from each other in any convenient manner, the occurrence of a low resistance in shunt to the resistance 91 can readily be avoided.

In Fig. 5, I have illustrated, more or less diagrammatically, a further modification or the arrangement of Fig. 2 in which it is not necessary to insulate the networks 92 and 93 from each other in guarding against the establishment of a leakage path about the resistance 91. In this form of my invention the junctions I02 and I03 of the networks 92 and 93 are directly connected by the conductor 94, and the condenser means for producing the desired resetting operations of the apparatus have been connected between the contact I0 and the high resistance 91. With this arrangement it will be apparent that it is not necessary to insulate the networks 92 and 93 from each other since they are directly connected by the conductor 94. Thus, the prevention of the occurrence of a low resistance shunt'about the resistance 91 is greatly simplified.

ihe condenser means employed in Fig. 5 for effecting the desired resetting adjustments of the apparatus have been illustrated as a plural stage condenser unit and may be identical with the plural stage condenser unit described in connection with Fig. 4.

In this embodiment of my invention the amplifier 33a has been dispensed with and a simplified relay arrangement employed instead for controlling the selective energization of a reversible motor to effect the desired circuit rebalancing and fuel valve adjustments. As illustrated, the valves 99 and I00 are adapted to control the selective energization of a pair of relay coils I32 and I33 and thereby the adjustment of a movable contact I34 into engagement with one or the other of a pair of contacts I35 and I39. The contact I34 is carried by a lever I31 which is pivoted at a point I39 and also carries a pair of armatures I39 and I which extend part way into the relay coils I32 and I33, respectively. The armatures I39 and I40 are connected to the lever I31 at points on opposite sides of the pivot point I38 so that when one or the other of the relay coils I32 and I33 is energized to a greater extent than the other, the associated armature will be pulled farther into the coil and rotate the lever I31 about its pivot point to thereby eflect movement of the contact I34 into engagement with one or the other of the contacts I35 and I36. When both of the'reiay coils are deenergized, the lever I31 is biased by gravity into a position wherein the contact I34 is intermediate 1 the contacts I35 and I35. In this embodiment.

anode voltage is supplied the valves 99 and I00 and thereby the coils I32 and I33 from the secondary winding I4I of the transformer I01.

The contacts I35 and I38 are each connected to a terminal of a pair of windingsI42 and I43, respectively, of a reversible electrical motor I44. The other terminals of the motor windings I42 and I43 are connected together and to the alternating current supply conductor L and the contact I34 is connected to the supply conductor L so that on engagement of the contact I34 with one or the other of the contacts I35 and I39, an energizing circuit will be closed to 2- corresponding winding I42 or I43 of the motor. The closure of that circuit will produce rotation of the motor in one direction or the other and thereby adjusti ment of the contact I0 and the fuel valve 2.

In Fig. 6 I have illustrated more or less diagrammatically another modiflcation o! the arrangement of Fig. 2 in which the energizing voltage for the amplifier 33a is supplied from the secondary winding I45 of the transformer I06 and in which a bridge network 92a is employed in lieu of the bridge network 92. In the modification of Fig. 6, as well as in the modification of Fig. 5,.it is not necessary to insulate the networks 92a and 93 in guarding against the establishment or a leakage path about the high resistance 91. As illustrated, the bridge network 82a is simpler in form than the corresponding network 92 of Fig. 2 in that the resistances and 9I of the latter have been dispensed with. The network 92a is comprised of only the transformer secondary winding I04 and the slidewire resistance 1 which is connected across the terminals of the winding I04. A center tap I02 on the transformer winding I04 is connected by the conductor 94 to the junction I 03 01 the bridge network 83.

In this arrangement, as in the apparatus 01' Fig. 5, the condenser 95 for controlling the resetting operation of the system is connected between the contact I and the high resistance 91. The junction of the condenser 95 and the high resistance 91 is connected by a conductor 32 to the control electrode of the valve I00. As illustrated a resistance I46 may desirably be inserted in the conductor 32 to prevent the discharge of the condenser 95 by the flow of grid currents. in the valve I 00.

While only a single condenser 95 has been employed for effecting the resetting operation of the apparatus, it will be apparent a plural stage condenser unit as shown in Fig. may be employed instead. Furthermore, while no provisions have been made in the arrangement of Fig. 6 for efi'ecting an initial magnified correction of the fuel valve 2, it will be plain that such provisions may be added in the manner described in connection with one or the other of Figs. 1-5, if desired.

In Fig. 7 I have illustrated, more or less diagrammatically, a modification of the control apparatus of Fig. 5 in which the bridge networks 92 and 83 of the latter arrangement have been replaced by a single bridge network 8Ia. The bridge network 8Ia includes the slidewire resistance "I and a pair of resistances I41 and I48 in two of its opposed arms, and the slidewire resistance II and a pair or resistances I49 and I50 in its remaining arms. A resistance I5I is connected in parallel to the resistance 1, and a resistance I52 is connected in parallel to the slidewire resistance II. The resistances I5I and I52 are simultaneously adjustable in opposite directions by a knob I53 and are provided for eflecting Energizing voltage is supplied the bridge network 8Ia from the secondary winding I of a transformer I55 having another secondary winding I56 and a line voltage primary winding I51 connected to the alternating current supply conductors L and L A rectifier I09 is connected in one energizing connection to the bridge network and a condenser I58 is connected across the energizing terminals of the network so that substantially steady direct current is supplied the latter.

Unbalance of the network 8I a may be detected by a single stage amplifier arrangement as shown in Fig. 5 to control the selective energization of a pair of relay I32 and I33, if desired, but in this form of my invention I have illustrated a two stage amplifier for that purpose. As illustrated, the relays I32 and I33 are connected in the output circuits or electronic valves I59 and I60, respectively, and the input circuits of the latter are adapted to be controlled by the output circuits of the valves 99 and I00. To this end, the output circuits of the valves 99 and I00 are coupled to the input circuits of the valves I59 and I60 in a manner identical to the coupling between the valves 99 and I00 and the valves I and I2I of Fig. 4.

Anode-voltage is supplied the valves 99, I00, I59 and I60 from the transformer secondary winding I56 and, as shown, the pairs of valves 99, I00 and l59, I60 are connected in inverse relationship to each other whereby the said pairs of Valves will be conductive during alternate half cycles or the supply voltage.

Since an additional stage of amplification is employed in this form of my invention, it will be apparent that the relays I32 and I33 may be larger and more rugged in construction than the correspondingly identified parts of Fig. 5 although the operation of the relays is the same in one direction or the other in accordance with the direction of unbalance of the network 8 la.

The provisions for effecting the desired resetting and initial magnified correction adjustments of the fuel valve 2 in this embodiment include a resistance I6I, connected between the energizing terminals of the network 8Ia, and the elements 95, 91, H5 and H6 which are connected between the contact I0 and a center tap on the resistance I6I. The connection and operation of the elements 95, 91, H5 and H6 isidentical to that of the correspondingly identified parts of Fig. 5 and hence further description is not believed necessary.

In this embodiment of my invention I have illustrated an alternative arrangement to the shown in Fig. 3 for arresting the resetting operation,. of the condenser 95 when the contact 6 assumes a predetermined low temperature position which it does not reach during the ordinary operation 01" the system and normally assumes only when the furnace is being started or shut down. This alternative arrangement referred to includes a normally open switch I62 connected to the terminals of the condenser 95 and which is adapted to be actuated into a closed position by an arm I63 carried by the Bourdon tube 4,

- as shown, when the contact 6 reaches a preand selectively controls the motor I f0r rotation determined low position. The switch I62 comprises a stationary contact connected to the lower terminal of the condenser as seen in Fig. 7 and a movable contact connected to the other condenser terminal and carried by a. flexible arm I64. A hook I65 at the upper end of the arm IE3 is adapted to engage the flexible arm I66 when the arm I63 reaches a predetermined position in its downward movement and moves the movable contact of the switch I62 into engagement with the stationary contact to thereby close a low resistance shunt about the condenser 95. The arm I63 is desirably made in two sections held together by an expansion joint. which may be of any well known form, so as to permit further downward movement of the contact 9 when the switch I62 is in its closed position.

In Fig. 8 I have illustrated, more or less diagrammatically, a modification of the arrangement of Fig. 7 in which a simplified bridge net'- work 8Ib energized from direct current supply conductors L and L is employed. As shown, the bridge network 8Ib includes a slide wire resistance I in two of its opposed arms and a slide wire resistance II in the remaining arms. A resistance I6I having a center tap is connected to the energizing terminals of the bridge network 8Ib and has its center tap connected through the resistance 91 and condenser 95 to the contact I0 as in the arrangement of Fig. 7.

A voltage divider I60, of which the slide wire resistance I comprises an extension, Is connected between the direct current supply conductors L and L and supplies energizing current to the electronic valves 99 and I00 in addition to the bridge network 8Ib. The contact 6 is connected to the negative terminal of the biasing resistance I M in the cathode circuits of the valves 99 and I00, to which terminal the control electrode of the valve 99 is also connected. The control electrode of the valve I00 is connected to the junction of the condenser 95 and the resistance 91.

The output circuits of the valves 99 and I00 are connected to the input circuit of an electronic amplifier 33a which may be identical to the amplifier 33a shown in Fig. 4. The only dlfierence in this arrangement over that disclosed in same, however, so that when the conductivity of the valve Hill of Fig. 8 rises above or falls below that of the valve 99, an alternating current in phase with or 180 out of phase with the voltage of the supply conductors L and L will be supplied the winding 36 of motor 9 to thereby produce rotation of the motor in a corresponding direction. I

It will be apparent that with the arrangement of Fig. 8, the current in the cathode circuits of the valves 99 and Hill will flow through the lower portion of the slide wireresistance i. In some cases this may be objectionable in that such cathode current flows may disturb the balance of the bridge network lib and will result in inaccuracies in the control operation of the apparatus.

In-Fig. 9 I have illustrated more or less diagrammaticallyja further modification of the arrangement of Fig. 7 in which the cathode current flows through the valves .8 and iilii are shunted about the bridge network lie so that the objectionable feature referred to above in connection with Fig. 8 is avoided. As illustrated the bridge network lia may be identical to the corresponding part of Fig. 7 but is supplied with energizing current from the direct current supply conductors L and L As shown current limiting resistances iii! and it! may be connected in the energizing connections to the bridge network lid. The electronic valves 9! and I" are connected to the input circuit or the amplifier 33a in a manner identical to that described in connection with Fig. 8.

In Fig. 9 I have illustrated an arrangement alternative to the arrangements shown in Figs. 3 and '1 for arrestingthe resetting operations of the condenser 95 when the contact 6 assumes a predetermined low temperature position which it does not reach during the ordinary operation oi. the system and normally assumes only when the furnace is being started or shut down. This alternative arrangement includes a switch it! which may be identical to and actuated in the same manner as the corresponding switch I62 of Fig. 7. The contact of switch I" carried by the flexible member I64 is connected in Fig, 9 to the terminal of the condenser l! remote from the resistance 91 and is also connected by the conductor 32, in whicha resistance iiia is inserted, to the contact II. The resistance iOa may be suitably high in value, but is appreciably smaller in value than the resistance 01. The stationary contact of the switch I6! is connected by a low resistance conductor to a contact la which is adjustable along the slidewire resistance 1, as, for example, by a knob'lb.

As will become apparent, when the switch in is actuated into its closed position, the potential on the condenser s! will build up to or fall oil to the potential difference existing between the contact ia and the network junction iiia and will be maintained at that potential irrespective of the position of the contact iii. This eitect is obtained because of the presence of the resistance 'iOa in the connection or the contact i. to the condenser IS. The potential so maintained adjusted to a value corresponding to the load on the furnace when the latter is being started up so that when the furnace temperature has reached the desired value the contact i0 and the fuel valve 2 will have been adjusted to their correct positions to maintain the furnace temperature at that value.

In Fig. 10 I have illustrated one form of control apparatus for use in conjunction with and to provide parts of the control circuit arrangement of Fig. 9. It is to be understood, however, that most of the apparatus parts shown in Fig. 10' may be used with the other control circuit forms of the invention. The apparatus shown in Fig. 10 comprisesa control panel it! in which is mounted a control instrument I10 for measuring the furnace temperature control quantity and for adjusting the contact 8 and the switch i" of Fig. 9. -In addition panel I68 provides a support for all oi the control apparatus shown in Fig. 9 except the motor 9 and the associated balancing resistance ii of the bridge network 8 la, both of which ordinarily must be located adjacent to the fuel valve 2 or other device controlled and at a distance from the control apparatus shown in Fig. 10. As shown the control instrument H0 is of the commercial Brown potenon the condenser 98 may be adjusted as desired,

simply by adjustment of the knob 112.

An important advantage of this arrangement is that the potential on the condenser ll may be vanometer.

tiometer type including a movable carriage ili which is deflected in accordance with variations in the quantity measured by means of a screw shaft 112. The latter is rotated in one directionv or the other by the rebalancing mechanism of the instrument in response to deflections of the pointer ill of a galvanometer which responds to unbalance in the potentiometer measuring system. When such a potentiometric instrument is used in lieu of the simple Bourdon'tube type of thermometer of Fig. 9, the furnace temperature responsive bulb 3 is ordinarily replaced by a thermocouple and the galvanometer responds to unbalance between the thermocouple voltage and a the voltage drop in the variable portion or the potentiometer slidewire resistance portion connected in series with the thermocouple and gal- Further description or illustration of the measuring and 'rebalancing features of the instrument I" is unnecessary, as they form no part of the present invention and in respect to those features the instrument iiil is oi well known commercialtype and generally alike or equivalent in principle to the control potentiometer instrument shown in Patent 1,946,280,

granted to me February 6, 1934. i

The Brown potentiometer control instrument movable carriage iii gives movements to a pen I", tracing a record of the value or the quantity measured on a chart "5 and cooperates for control purposes with a control table I". The latter is located at a point along the path of .travel of the pen carriage i1i which may be adjusted and ilxes the normal or desired value or the controlled quantity measured by the instrument. A part i'll carried by the control table I16 and a cooperating part I" of the pen carriage iii cooperate to adjust a pivoted part i1! angularly about its pivotal axis I" into'different positions corresponding to diilerent distance", of departure of the controlled quantity from the desired or normal value thereof.

As shown in Figs. 11 and 12 member H9 is connected by a link i8i to the arm in of a rock shaft ill which is thereby given angular adjustments corresponding to those. given'the part H9. The rock shaft it! supports and moves arms which support or carry the contact 8 and the arm I63 of Fig. 9 which engage the slide wire resistance I and the flexible arm I 64 of switch m, respectively. I

The panel I69 may desirably support at its rear side and below the instrument I10 the supporting chassis of the amplifier 33a. Mounted on the panel IE9 at the front side of the panel I" are the previously mentioned switch 56a and the knobs or dials lb, 98, H1, Illa and I53 for effecting the various apparatus adjustments.

As will be apparent to those skilled in the art, the instrument I68 of Figs. 1012 may be replaced by an instrument of another known or suitable form adapted to measure a control quantity and directly or through suitable relay mechanism to give corresponding adjustments to the contacts 6 and I82. The control quantity is not necessarily temperature but may be a pressure, velocity or other force susceptible of measurement by the instrument and a measure of which may be advantageously utilized in effecting a control operation.

While in accordance with the provisions of the statutes, I have illustrated and described the best form of rm; invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit ofmy invention as set forth in the appended claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a control system, a normally balanced electrical network, means to unbalance said network in response toa condition change, control means responsive to unbalance of said network to control said condition in the direction to counteract said change and including follow-up means to rebalance said network on unbalance thereof, and reset means included in said network and operative only while said condition is departing from a predetermined value to further unbalance said network in the same direction.

2. In a control system, an electrical network,.

means to adjust said network in response to a condition change, control means responsive to said network adjustment to control said condition in the direction to counteract said change and thereby stabilize said condition, and reset means included in said network and operative only while said condition is departing from a predetermined value to further adjust said control means in the same direction and thereby restore said condition to a predetermined value.

3. In a control system,the combination of a device to be positioned to a plurality of positions for controlling the value of a condition, electrical control means the state of which is varied in accordance with changes in the value of the condition to be controlled and having a desired state corresponding to a desired normal value of the condition, means for positioning said device in accordance with the state of said electrical control means to maintain said condition at said desired normal value, reset means including an electrical reactance means to further vary the state of said electrical control means on departure of said condition, from said normal value, and means to short circuit said electrical reactance means to render said reset'means inoperative when the desired normal value exceeds a predetermined amount. 4. In a control system, two separate electrical bridges, an impedance and an associated element included in each of said bridges, said impedance and associated element being adapted for relative movement to form a control couple, an electrical connection between said bridges, means responsive to a control condition for relatively adjusting one of said control couples on a change in said condition, means connected to each of said bridges and responsive to a change in an electrical condition of said bridges produced by said adjustment for effecting a corrective action on said control condition in the direction to counteract said change and for effecting a corresponding'relative adjustment of the second said control couple to neutralize the change in the electrical condition of said bridges produced by the adjustment of said one couple, and means responsive to changes in the value of said control condition'irrespective of the actual value thereof to effect a further corrective action of a variable amount depending upon the rate of adjustment of said one control couple by said first mentioned means on said control condition in the same direction immediately upon change in trend of the value of said condition.

5. The combination of claim 4 wherein said last mentioned means include means to reduce the neutralizing effect of an adjustment of said second control couple.

6. The combination of claim 4 wherein said last mentioned means are electrical means and include electrical reactance means to reduce the neutralizing effect of an adjustment or said second control couple.

7. The combination of claim 4 wherein said last mentioned means are electrical means and include electrical capacitance means to reduce the neutralizing eiIect of an adjustment or said second control couple.

8. In a control system, a normally balanced electrical network, electrical current energizing means for said network, means to unbalance said network in response to a condition change, control means responsive to unbalance of said network to control said condition in the direction to counteract said change and including follow-up means to rebalance said network on unbalance thereof, and means controlled by said condition responsive means to vary said electrical current energizing means and thereby the energi'z'ation of said network to effect a further corrective action on said condition in the same direction.

9. In a control system, a normally balanced electrical network, means to unbalance saidnetwork in one direction or the other in response to a condition change, control means responsive to unbalance of said network to control said condition in the direction to counteract said condition change and including follow-up means to rebalance said network on unbalance thereof, and means operativeas an incident to actuation of said first mentioned means by a condition change to unbalance said network an additional amount in the same direction to thereby eifect an initial magnified corrective action of said control means to counteract said condition change and to later unbalance said network in a sense such as to reduce the corrective action of said control means when the rate of change of said condition as measured by said first mentioned means is lower than a predetermined finite value.

10. In a control system, a normally balanced electrical network, means to unbalance said network in response to a condition change, control means responsive to unbalance of said network to control said condition in the direction to counteract such change and including adjustable fol low-up means to rebalance said network on unbalance thereof, and means responsive to changes in the value of said condition to temporarily reduce the rebalancing eflect of adjustment of said follow-up means to effect a further corrective action on said condition in the same direction immediately upon change in thetrend of the value of said condition.

11. The combination of claim wherein said last mentioned means are physically stationary.

12. In a control system, means for producing an electromotive force representative of the magnitude of a variable condition to be controlled, an adjustable source of electromotive force,

means to oppose said electromotive forces to rective action on said control condition in the same direction.

15. The combination of claim 14 wherein said reset means are physically stationary.

16. The combination of claim 14 wherein said means responsive to changes in the value of the condition irrespective of the actual value thereof are physically stationary.

17. The combination of claim 14' wherein said reset means and said means responsive to changes in the 'value of the condition irrespective of the actual value thereof are physically stationary.

18. In a control system, a normally balanced electrical network, means to unbalance said network in response to a condition change, control means responsive to unbalance or said network -to control said condition in the direction to derive an electrical potential varying in accordance with the difference between said electromotive forces, means for controlling a corrective agent for said condition, a device under control of said potential to control said last mentioned means and to adjust said second mentioned electromotive force to balance said first mentioned electromotive force and physically stationary means to temporarily reduce the balancing effect of adjustment of said second mentioned electromotive force.

13. In a control system, device adapted to be adjusted to diflerent positions in accordance with the changes in a variable condition to be trolled means adapted to be controlled in one sense or in an opposite sense, adjustable means to control said electrically controlled means, means through which a movement or said device from one position to another adjusts said adjustable means to control said electrically controlled means in a sense and to an extent depending upon the direction and magnitude of the movement, adjustable means adjusted by said electrically controlled means tending to neutralize the adjustment of said adjustable means, and means reducing the neutralizing eilect or adjustment of .said last mentioned adjustable means a variable amount depending upon the rate of v adjustment of said device.

14. In a control system, two separate electrical bridges, an impedance and an associated element included in each of said bridges, said impedance and associated element being adapted for relative movement to form a control couple, an electrical connection between said bridges, means responsive to a control condition for relatively adjusting one of said control couples on a change in said condition, means connected to each of said bridges and responsive to a change in an electrical condition of said bridges produced by said adjustment for effecting a corrective action on said control condition in the direction to counteract said change and for effecting a corresponding relative adjustment of the second said control couple to neutralize the change in the electrical condition or said bridges produced by the adjustment of said one couple, means responsive to changes in the value of said control condition irrespective of the actual value thereof to effect a further corrective action on said control condition in the same direction immediately upon change in trend of the value of said condition, and reset means to the combination of a controlled, electrically con-- slowly elect a further corcounteract said change and including follow-up means to rebalance said network on unbalance thereof, means responsive to changes in the value of said condition to effect a further corrective action on said condition in the same direction immediately upon change in the trend of the value of said condition, and reset means jointly controlled by said first mentioned means and by said control means to slowly eiiect a further corrective action on said condition in the same direction.

19. The combination of claim 18 wherein said reset means is adapted to effect such further corrective action on said condition only when said condition is departing from a predetermined value.

20. In a control system, a normally balanced electrical network, means to unbalance said network in response to a condition change, control means responsive to unbalance of said network to control said condition in the direction to counteract said change and including follow-up means to rebalance said network on unbalance thereof, means responsive to changes in the value of said condition to effect a on said condition in the same direction immediately upon change in the trend of the value of said condition, and physically stationary reset means to slowly effect a further corrective action on said condition in the same direction.

21. In a control swstem, a normally balanced electrical network, means to unbalance said network in response to a condition change, control means responsive to unbalance of said network to control said condition in the direction to counteract said change and including adjustable follow-up means to rebalance said network on unbalance thereof, means responsive to changes in the value of said balancing effect of adjustment of said follow-up means to eflect a further corrective action on said condition in the same direction immediately upon change in the trend of the value of said condition, and reset means to slowly efiect a further corrective action on said condition in the same direction.

22. The combination of claim 21 wherein said reducing means are physically stationary.

23. The combination or claim 21 whereinjsaid reset means are physically stationary.

24. The combination of claim 21 wherein said reducing means and said reset means are physically stationary.

25. In a control system the combination of a further corrective action 1 condition to reduce the re- 

